This invention relates to an aluminum oxide substrate material for a magnetic head and to a method for producing the same. The product is improved for high density recording by using a thin film magnetic head. Magnetic head material for high density recording should not have a lower magnetic permeability in the higher frequency range, and it is hard to make a narrow track in the usual soft ferrite material. For the substrate material for thin film magnetic heads, the complete absence of minute pores on the super precise mirror-like finished surface, and easy machinability are required. Generally, a ceramic substrate is recommended for a thin film magnetic head. The ceramic powder has to be sintered and formed until its theoretical density is substantially reached, and strong binding forces and fine structure in the ceramic crystal must be present to obtain a super precise mirror-finished base sheet without minute pores. On the other hand, the machinability of such ceramic sintered bodies is very difficult.
Heretofore, hot-pressing methods and hot isostatic pressing methods (hereinafter called the HIP method) have been proposed as effective sintering methods for producing such ceramic base sheet. In the hot pressing method, Al.sub.2 O.sub.3 -TiC raw material powder is hot pressed at 1600.degree. C. to 1800.degree. C. to produce the sintered body. In the process of sintering, less than 0.5 percent by weight of one or more components selected from the group consisting of MgO, NiO and Cr.sub.2 O.sub.3 is/are usually added to the green compact to inhibit the grain growth of the sintered body during the sintering, because the sintering must be conducted at a relatively high temperature, but it is still not sufficient to obtain a sintered body having a theoretical density and there are still problems with machinability.
Referring now to the HIP method, which has come to the stage of practical application, the green compact must be formed, as a preliminary treatment, into a presintered compact so that the compact has a density of more than 94% of the theoretical density. In the production of such Al.sub.2 O.sub.3 -TiC ceramic base sheet to which the present invention is directed, less than 1.0 percent by weight of one or more components selected from the group consisting of MgO, NiO and Cr.sub.2 O.sub.3 is/are usually added to the green compact to inhibit the grain growth of the sintered body.
However, such addition of the grain growth inhibitor is still not sufficient to obtain a presintered body having the aforementioned intended relative density, unless the presintering is effected at an elevated temperature of 1850.degree. C. to 1950.degree. C.
Then, however, since an Al.sub.2 O.sub.3 -TiC ceramic base sheet is exposed to the high temperature during the presintering operation, Al.sub.2 O.sub.3 grains and TiC grains grow in spite of the addition of the above-mentioned grain growth inhibitor. In the super precise mirror-finish base sheet, such grains often result in minute pores and chipping-off of the the edge when the sheet is processed mechanically to provide a track.
Furthermore, since the above production process necessitates presintering at the high temperature, it gives rise to an energy saving problem.